The fifth generation of mobile telecommunications and wireless technology is not yet fully defined but in an advanced draft stage within 3GPP. 5G wireless access will be realized by the evolution of Long Term Evolution, LTE, for existing spectrum in combination with new radio access technologies that primarily target new spectrum. Due to the scarcity of available spectrum, spectrum located in very high frequency ranges (compared to the frequencies that have so far been used for wireless communication), such as 10 GHz and above, are planned to be utilized for future mobile communication systems. Thus, evolving to 5G includes work on a New Radio (NR) Access Technology (RAT), also known as 5G or next generation (NX). The NR air interface targets spectrum in the range from sub-1 GHz (below 1 GHz) up to 100 GHz with initial deployments expected in frequency bands not utilized by LTE. Some LTE terminology is used in this disclosure in a forward looking sense, to include equivalent 5G entities or functionalities although a different term may be specified in 5G. A general description of the agreements on 5G New Radio (NR) Access Technology so far is contained in 3GPP TR 38.802 V0.3.0 (2016-10), of which a draft version has been published as R1-1610848. Final specifications may be published inter alia in the future 3GPP TS 38.2** series.
Physical resources for RATs used in wireless communication networks may be scheduled in time and frequency in what could be seen as a time and frequency grid. For example, the basic downlink physical resource of the RAT LTE can be seen as a time-frequency grid as illustrated in FIG. 1. LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and a pre-coded version of OFDM called Single Carrier Frequency Division Multiple Access (SC-FDMA) in the uplink. LTE uses OFDM to transmit the data over many narrow band carriers, usually of 180 KHz each, instead of spreading one signal over the complete 5 MHz carrier bandwidth, in other words OFDM uses a large number of narrow sub-carriers for multi-carrier transmission to carry data. OFDM is thus a so called multi carrier system. Multi carrier systems are systems that uses multiple sinusoidal waves of predefined frequencies as multiple subcarriers. In multicarrier systems, data are divided on the different subcarriers of one transmitter. The difference between the frequencies of two adjacent subcarriers is called the frequency domain subcarrier spacing or subcarrier spacing for short. The OFDM symbols are grouped into so called physical resource blocks (PRB) or just resource blocks (RB). The basic unit of transmission in LTE is a RB, which in its most common configuration consists of 12 subcarriers and 7 OFDM symbols (one slot). In LTE the resource blocks have a total size of 180 kHz in the frequency domain and 0.5 ms (one slot) in the time domain. Each element in the time-frequency grid containing one symbol and one subcarrier is referred to as a resource element (RE). Each 1 ms Transmission Time Interval (TTI) consists of two slots (Tslot), usually represented by 14 OFDM symbols. LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms, as shown in FIG. 2. The resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
The new RAT NR will use a similar structure for the physical resources as LTE, using multiple carriers in frequency and symbols in the time domain, defining resource elements of physical resource blocks. The physical resource parameters may vary in NR. For example, the carriers may span a variable frequency range, the frequency spacing or density between the carriers may vary, as well as the cyclic prefix (CP) used. The frequency spacing between subcarriers can be seen as the frequency bandwidth between the center of a subcarrier and the adjacent subcarrier, or the bandwidth occupied by each subcarrier in the frequency band.
A numerology defines basic physical layer parameters, such as subframe structure and may include transmission bandwidth, subframe duration, frame duration, slot duration, symbol duration, subcarrier spacing, sampling frequency, number of subcarrier, RB per subframe, symbols per subframe, CP length etc. In LTE the term numerology includes, e.g., the following elements: frame duration, subframe or TTI duration, slot duration, subcarrier spacing, cyclic prefix length, number of subcarriers per RB, number of RBs within the bandwidth (different numerologies may result in different numbers of RBs within the same bandwidth).
The exact values for the numerology elements in different RATs are typically driven by performance targets. For example, performance requirements impose constraints on usable subcarrier spacing sizes, e.g. the maximum acceptable phase noise sets the minimum subcarrier bandwidth while the slow decay of the spectrum (impacting filtering complexity and guardband sizes) favors smaller subcarrier bandwidth for a given carrier frequency, and the required cyclic prefix sets the maximum subcarrier bandwidth for a given carrier frequency to keep overhead low. However, the numerology used so far in the existing RATs is rather static and typically can be trivially derived by the UE, e.g., by one-to-one mapping to RAT, frequency band, service type (e.g., Multimedia Broadcast Multicast Service (MBMS)), etc.
In LTE downlink which is OFDM-based, the subcarrier spacing is 15 kHz for normal CP and 15 kHz and 7.5 kHz (i.e., the reduced carrier spacing) for extended CP, where the latter is allowed only for MBMS-dedicated carriers.
The support of multiple numerologies has been agreed for NR, which numerologies can be multiplexed in the frequency and/or time domain for the same or different UEs. In NR which is to be based on OFDM, the multiple numerologies will be supported for general operation. A scaling approach (based on a scaling factor 2{circumflex over ( )}n, n∈N_0) is considered for deriving subcarrier spacing candidates for NR. Values for subcarrier bandwidths currently discussed include among others 3.75 kHz, 15 kHz, 30 kHz, 60 kHz. The numerology-specific slot durations can then be determined in ms based on the subcarrier spacing: subcarrier spacing of (2{circumflex over ( )}n*15) kHz, m being an integer, gives exactly ½m 0.5 ms for a slot that is 0.5 ms in the 15 kHz numerology. Subcarrier spacings of at least up to 480 kHz are currently being discussed for NR (the highest discussed values correspond to millimeter-wave based technologies). It has also been agreed that multiplexing different numerologies within a same NR carrier bandwidth is supported, and FDM and/or TDM multiplexing can be considered. It has further been agreed that multiple frequency/time portions using different numerologies share a synchronization signal, where the synchronization signal refers to the signal itself and the time-frequency resource used to transmit the synchronization signal. Yet another agreement is that the numerology used can be selected independently of the frequency band although it is assumed that a very low subcarrier spacing will not be used at very high carrier frequencies.
In NR the transmission bandwidth of a single carrier transmitted by a network node (also known as gNB) may be larger than the UE bandwidth capability, or the configured receiver bandwidth of a connected device (such as UE). Each gNB may also transmit using different numerologies which are time division multiplexed (TDM) or frequency division multiplexed (FDM).
Reference signals, also known as “pilots” or “pilot signals”, can be used in wireless communication for estimating the properties of a radio channel. Reference (pilot) signal aided channel estimation is a widely used technique to enable wireless access points and UEs to acquire channel state information at the transmitter and/or receiver (CSIT/CSIR). When multiple UEs are multiplexed on the same or overlapping time and frequency resources using spatial multiplexing, orthogonal reference or pilot sequences are used to acquire CSIT/CSIR for each UE. By orthogonal sequences is meant sequences that are basically non-overlapping, uncorrelated, or independent in a mathematical sense and, when transmitted as signals, do not interfere with each other.
To ensure a high degree of orthogonality among the UEs, sufficiently long reference or pilot sequences must be used. In systems with spatial multiplexing of multiple wireless devices or UEs, such as Multi-user multiple-input and multiple-output (MU-MIMO) systems, orthogonal reference sequences allow multiple users (UEs) to be spatially multiplexed on the same or overlapping time/frequency resources as long as the reference sequences of these UEs can be separated in the code domain. The number of orthogonal reference sequences and thereby the maximum number of MU-MIMO users (UEs) is limited by the length of the reference sequences.
For example, in the uplink of LTE systems, UEs use orthogonal demodulation reference signals (DMRS) to enable the base station (BS) to acquire CSIR. The DMRS is constructed by means of cyclically shifted Zadoff-Chu sequences that are mapped on predefined resource elements (subcarriers) of specific single carrier frequency division multiplexed (SC-FDM) symbols within the physical resource blocks (PRB) on which the UE is scheduled.
A fundamental trade-off is related to the number of resource elements (complex symbols) used for reference sequence construction and transmission and the resource elements available for data symbols within the fixed number of totally available resource elements within the PRB on which the UE is scheduled. When BSs are equipped with a large number of antennas, they can support the spatial multiplexing of many users provided that these users can be assigned near orthogonal reference (pilot) sequences. In multi carrier systems, such as OFDM for multicell MU MIMO systems, a large number of users may be served simultaneously in neighbor cells. When the length and thereby the number of distinct reference (pilot) sequences are constrained, the reference (pilot) sequences must necessarily be reused by neighboring BSs. These systems are casually referred to as “pilot reuse-1” systems, since the same set of reference (pilot) sequences are applied in neighboring cells. In pilot reuse-1 systems, when multiple UEs use the same reference (pilot) sequence, they cause interference to each other at the BS. This interference is often referred to as pilot contamination (PC). Pilot contamination degrades the quality of channel estimation (both for CSIT/CSIR) which in turn leads to degraded Uplink/Downlink (UL/DL) throughput.
There is thus a need to provide longer reference sequences to be able to serve more simultaneously spatially multiplexed users using orthogonal sequences.